This paper describes LIMSI participation to the WMT'14 Shared Task on Quality Estimation; we took part to the wordlevel quality estimation task for English to Spanish translations. Our system relies on a random forest classifier, an ensemble method that has been shown to be very competitive for this kind of task, when only a few dense and continuous features are used. Notably, only 16 features are used in our experiments. These features describe, on the one hand, the quality of the association between the source sentence and each target word and, on the other hand, the fluency of the hypothesis. Since the evaluation criterion is the f 1 measure, a specific tuning strategy is proposed to select the optimal values for the hyper-parameters. Overall, our system achieves a 0.67 f 1 score on a randomly extracted test set.